This invention relates to the use of volume optical filters which have sharp absorption edges but shown no directional dependence of absorption or overall intensity. More particularly, this invention relates to a volume filter which is used to eliminate an undesirable set of hyperfine lines (and thereby increase the signal to noise ratio) in an isotropically sensitive atomic resonance filter-detector.
Ultra high-Q isotropically sensitive atomic resonance filter-detectors (also known as QLORD filters) such as described in U.S. Pat. No. 4,292,526, the entire contents of which is incorporated herein by reference, utilize atomic resonance transitions. These filter-detectors are bounded by two optical bandpass filters with a wide field of view. The filter-detector itself has a wide field of view (2.pi. steradian) and a very narrow acceptance bandwidth (Approx. 0.01.ANG.). The outer and inner filter have no common transmission band, so that the incoming light is admitted and excites the atomic vapor. The vapor then emits at a different wavelength, passes through the second filter, and is detected. Absorption can take place in more than one set of hyperfine lines, so that extraneous optical noise from one set of lines can interfere with the signal received through the other.
Accordingly, there is a need to eliminate the undesirable set of hyperfine lines. Unfortunately, conventional blocking filters have edges that are not sharp. Therefore, they would be of little use in selectively attenuating closely spaced lines. On the other hand, notch filters or attenuators would be of little use even with their sharp absorption edges, because of the strong angle dependence of absorption. Light entering such systems could not be from a diffuse source, but only from a very narrow acceptance angle. This is the reason that Lyot or birefringent filters have proven impractical for such applications.
As mentioned, the presence of unwanted hyperfine lines admits extraneous optical noise into the system. Since the noise in the signal to noise ratio is the square root of the noise from a given source, the elimination of one of two sets of hyperfine lines of equal strength would reduce optical noise by some 35%.